This invention relates to analog to digital converter circuits and more particularly to MOS analog to digital converter circuits including enhanced capacitors.
The problem of converting analog signal values to an equivalent digital representation appropriate for various digital computing or digital readout requirements is basic to the digital computer field. Numerous prior art solutions to this problem have been proposed using a wide variety of circuit techniques.
Although monolithic integrated circuits using MOSFET devices have been widely used to implement a wide variety of complex digital functions, they have not been extensively utilized to fulfill the requirements of analog to digital converter circuits because of present wide tolerance variations in MOSFET device characteristics associated with present manufacturing methods. One approach to analog to digital conversion has been to provide a common analog input to a plurality of digital comparator circuits in which each of the comparator circuits has a separate input connection to a reference voltage. The different individual reference voltages thus partition the analog input voltage into different ranges depending upon whether the analog input signal is above or below a particular reference voltage value. These ranges are then correspondingly represented by the digital outputs of the comparators. A direct connection of the analog input signal to the inputs of digital comparator circuits suffers the fundamental disadvantage that the connection cannot be interrupted for operation on a sample mode basis. Also, a direct connection precludes the use of temperature or voltage compensation via series and shunt elements.